1. Field of the Invention
The present invention relates to bias current control in an electrophotographic type image forming apparatus.
2. Description of the Related Art
Recently, image forming apparatuses, which use a laser scanning optical system in an exposure apparatus, have used a vertical cavity surface emitting laser (VCSEL) as a light source of the exposure apparatus to improve productivity and resolution thereof. In the VCSEL, it is difficult to extract rear light, and thus light amount control is generally executed using front light. For example, a beam splitter is used for the light amount control. A part of the front light of the VCSEL is reflected by the beam splitter, and a photodiode detects a light amount thereof. In the case of the light amount control using the beam splitter, the light amount to be detected by the photodiode is reduced by lowering reflectance of the beam splitter so that an adequate light amount is secured for exposure. If the light amount detected by the photodiode is small, an effect of a dark current of the photodiode cannot be ignored and will be an error with respect to a detected signal.
Thus, a dark current cancelling unit is required to remove an error caused by a dark current. For example, Japanese Patent Application Laid-Open No. 07-167613 discusses a configuration in which a differentiating circuit removes a voltage corresponding to a dark current generated in a position detection element.
However, the dark current cancelling unit may sometimes affect settings of a bias current of the VCSEL. To a laser light source, the bias current is applied to increase a switching speed of the laser light source. In addition, automatic light amount control (APC) (hereinbelow, referred to as bias APC) is executed to obtain a threshold current value for setting the bias current. FIG. 14A is a graph illustrating an operation of the bias APC. As illustrated in FIG. 14A, an intersection point of a straight broken line obtained by executing the automatic light amount control with an X axis is obtained as a threshold current value Ith. Laser oscillation is started near the threshold current value Ith, and a minute amount of a laser beam is output. Generally, the threshold current value is scarcely affected by the minute light amount. However, the minute light amount may sometimes affect the threshold current value if the dark current cancelling unit is used. For example, when a dark current cancelling circuit as illustrated in FIG. 13A is used, the threshold current value Ith is obtained as a threshold current value Ith′ due to Poffset which is influence of the minute light amount as illustrated in FIG. 14B. In the case in FIG. 14B, the obtained threshold current value Ith′ is larger than an actual threshold current value Ith, and the minute light amount further increases.
When an APC sequence before image formation (hereinbelow, an initial APC sequence) is executed, a capacitor is charged from 0 V (a ground voltage: GND). Thus, in an early stage of the initial APC sequence, a voltage of the capacitor is not stable, and it is difficult to secure accuracy of the threshold current value. In such a circumstance, there is an issue that when a correction coefficient α multiplied by a threshold current is set to a value α at the time of image formation, a value of a bias current Ith′ exceeds a true threshold current value Ith as illustrated in FIG. 14B, a value of the bias current cannot be accurately set.
The present invention is directed to restraining deterioration of setting accuracy of a bias current value in consideration of the above-described issue.